I am in the process of building a digital torque meter that Jake Palmer designed and I am going to document my steps for others to follow if they desire to build one. Jake's design uses 3D printed parts and the files can be found here:https://www.thingiverse.com/thing:2441787

The first photos is the 3D printed parts. I have a Prusa i3X printer, and printed it using PLA, 40% fill density, 0.2 mm layer thickness. The buttons and collars I printed together and used an external fan to cool them as printing. I found this produces better quality small sized parts. The lettering on the face plate I just filled with some spackle and wiped the excess off, this way the contrast in colors makes it easy to read. Once you have the parts printed then start by taking the scale apart.

Remove the battery cover. Then remove the two case screws on the right side (of the photo) and the two load cell screws near the center. The load cell screws will be tight from the factory. Use a plastic pry tool (you can get these at harbor freight or ebay or amazon, they are for removing car dashboard panels etc). Use the pry bar on the opposite end from the battery cover. Once it pops loose, carefully open it up as seen in the last photo.

Now remove the four screws holding the bracket to the load cell, and the four screws holding the green pcb/display to the case. SAVE these 4 screws!

Carefully flip it over and lift the silver weighing pan that is connected to the load cell. Using the pry bar, pry off they silver pan from the bracket. It is just held with double sided tape. Be careful not to damage the load cell (note how I am holding the pan when prying).

Remove the two screws to free the load cell. These screws will be tight.

Cut the two red wires from the battery case. You should have what you see in the last photo. The display,load cell, and the four small screws.

Now it time to start assembling. The first photos shows some items used in this step. I chose to print eh version that uses 1/4 x 1/8 teflon tube instead of the ball bearings. When I went to order the tube, McMaster Carr showed it was discontinued. So instead I ordered low friction dry running sleeve bearings made from Rulon J. They come in the same size and are precision machined. The OD is .251/.258" and the ID is .129/.131". Mcmaster Carr Part number 6377K49, and are $2.33 each. Two are required.

Also in that photo you see the shaft material I used. Since I was ordering from Mcmaster Carr anyhow , instead of 1/8" music wire I went with .125" rotary shaft material. It is 316 stainless so will not corrode, OD is ground so the tolerance is tight +0/-.0002" and has a very nice surface finish, all this will reduce friction. This material is not as hard as music wire or hardened shaft material whic is OK for this application as iot means the set screws used will bit into the shaft better. Mcmaster Carr part number 1263K39 cost was $9.29 for 12" (enough for 3 torque meters).

I also ordered the 2 x AAA battery holder. McMaster Carr part number 7712K17 cost was $1.48.

The holes in my case were close to .250 but I could not get the sleeve bearings to go in. Fortunately Dremel has a .250 stone bit that works well to ream the hole. I uses the tool with my hand to size the hole. Once I sized them the sleeve bearing pushed in snugly. I then ran the shaft material through to see how well they sleeves lined up and it was perfect. No friction.

I then put some silicone glue on the back of the black batter holder and glued it to the floor of the case. Make sure those red and black wires go through the opening.

Next photo is the back of the display board, The 4 wires on the left go to the load cell. The two red wires on the right are the battery connections. I decided to unsolder the four load cell wires and remove the two red battery wires. I suggest doing this only if you are good at soldering small items on circuit boards and have a soldering station with a fine tip. If not I suggest using a connector in the middle of the wires like Jake did on his (see the photo on his Thigiverse page).

Once the wires were removed, I then used those four small screws to attach the display to the back of the face place. Make sure to put those printed button in first! Carefully screw those four tiny screws in and don't over torque or you may break the plastic stud off or strip it out.

Flip it over and check that buttons work when you press on the, they should move with no sticking.

I then installed the load cell using two 3mm x 25mm screws from underneath. Tighten these until snug. You don't want the load cell to move in operation. Notice the four wires are feed though the hole in the case to the front where the display will be.

I then put the case on its side and the display next to it as seen in the photo and resoldered the four load cell wires and the battery box wires. Note the polarity on the battery wires (the pcb is labeled V- and V+). The display was then attached to the case using four 3mm x 6mm button head screws. You can also use the four 3mm x 6mm screws that held the load cell to the weighing pan during the scale disassembly. There was no need for me to tap these holes for the thread. The hole size was just right that the screws self tapped when threading. Since this is plastic no need to go overboard torquing these screws down. I then took off the screen protector from the leftover scale parts (it peels off) then attached it to the face plate to nice finished look.

The last picture in this post is my testing the unit to make sure it powers on OK. I found that the back light intensity is based on the strength of the battery. I had some old batteries I threw in and the scale worked but the backlight was very dim. New batteries made it bright and easy to read.

Up next are the hooks. I printed a couple of the hook collars as I wanted a thicker wire hook (.039 wire) for heavier motors and a thinner wire (.025) for events like F1R, ministick etc that use small o-rings. I printed all three size Jake had with his file. The largest hole collar printed the hole all the way through the part. I did have to size it with a 1mm drill. The middle size collar part of a hole on the first layer so I had a reference point to drill my .025 hole, the third one printed no hole as the diameter is just too small for my printer to handle. What you do is bend your favorite hook shape, then install into the collar, then make a 90 degree bend to fit the wire in the slot that is on the inside.

The metal stop collar fits right inside and you have to replace the short 1/8" long set screw with one 3/16" or longer. I happened to have 1/4" long ones so used those.

Next I cut the shaft to 2 7/8" long with my dremel. I also bought a few #6-32 x 1" long screws at my local Ace hardware store and the washers and nuts needed to make the arm that touches the load cell. The first one I made was using the parts suggested by Jake, and it works out just like he says on his Thigiverse page. I then decided I wanted to make a special washer from some thin sheet metal stock. The fender washer is around .040" and I wanted it thinner so it would have a very precise point of contact. The metal I used is .010" thick and is from a small apple juice can. It is a mild steel compared to thin aluminum on pop cans. I cut the washer out using a 7/16" diameter hole punch and a 1/8" diameter hole punch (the punch set was bought from Harbor freight). The photo I have shows the thickness difference in the purchased washer compared to the one I made from the sheet metal.

I did a test fit of the parts and found that the washer comes very close to the front of the case. So I took one of the spare sleeve bearings I had and slid it on some spare shaft material then used a razor to cut a spacer about 1/16" wide. The Rulon J material cuts easy like teflon so you can get a clean cut. I then did a test fit up with the spacer to make sure it fit OK. .

This next step is my own addition due to me being paranoid. The hook the rubber attaches to is secured to the shaft by a set screw. I am paranoid the set screw will come loose and the hook will fly toward me some day. Especially as I plan to take the hook on and off depending on what event I am flying. So I used my dremel and made a slot on the shaft so they the set screw sits down in that slot. So there is both mechanical friction of the set screw plus a physical barrier for the set screw.

I also made a similar flat about .015" deep where the torque transfer arm is attached to the shaft. When the rubber is pulling this takes the pull load of the rubber and I am afraid it would come loose. This 6-32 screw does have a nice large head so you can really torque on it but the added groove give me peace of mind. Now adding that slot in this location means you have to make sure your washer is still 1" from the center of the shaft if you want accurate in-ounce readings. I used calipers to measure the washer position. If necessary you can file off the end of the 1" screw to get the correct length.

Once the flats were made the parts were reassembled. Now you will notice there is some shaft sticking out the back. In reality the shaft can be flush with the back of the case as the load is not carried at the back sleeve bearing. The front takes the load. But as an additional safety feature I added the rear collar to act as a safety net if ever the main collar should become loose and try to pull forward. In that case this rear one will stop it from going forward. The rear collar does not touch anything, there is a gap between it and the case. Yes I know it is redundant but years ago I had a solder joint break on a torque meter when winding and the parts flew right at me.

Next I cut the shaft to 2 7/8" long with my dremel. I also bought a few #6-32 x 1" long screws at my local Ace hardware store and the washers and nuts needed to make the arm that touches the load cell. The first one I made was using the parts suggested by Jake, and it works out just like he says on his Thigiverse page. I then decided I wanted to make a special washer from some thin sheet metal stock. The fender washer is around .040" and I wanted it thinner so it would have a very precise point of contact.

If, instead of the screw you attach the washer between two nuts (like I did in my original design a few years ago) you can adjust the arm length in high precision! Also note that the desired arm length (one inch in your casse I suppose) is the distance from the meter shaft to the washer edge touching the scale element, not the distance of the washer attachment point on the screw.

I am not sure what you are correcting in my post. I stated: "... you have to make sure your washer is still 1" from the center of the shaft if you want accurate in-ounce readings."

How is 1" from the center of the shaft to the washer that is touching the load cell incorrect information? There is a spacer washer between the bolt head to offset the transfer washer so it ends up 1" from the center line.

The 6-32 x 1" bolt measured .990" from the end to under the head, the spacer washer is .030 thick, my transfer washer is .010" so the center of the washer is .955" from the bolt end. Half the 1/8" shaft diameter is .0625 minus the -.015 flat I made = .0475" from shaft centerline to where the end of the screw will start. So .0475 +.955 = 1.0025" which is close enough to 1.000" for me.

Using Jakes original design. Assuming the same .990" length screw, same .030 spacer washer, and .040 thick transfer washer (fender washer) then .990-.03-.02(center of washer) +.0625 (half of 1/8")= 1.0025"

Sorry I cannot quite follow your description with my pidgin English. But you do not mention the washer diameter in the calculation, so I made the drawing below to make my case clear. In the picture, the circle on the left is the collar around the meter shaft, the rectangle to the right of it is the bolt/arm, and the narrow vertical rectangle is the washer. The two nuts holding the washer in place are omitted. With red arrow I drew the distance from the center of the shaft to the edge of the washer. That diagonal is the dimension which should be one inch.

This design has the screw/arm parallel to the load cell. So the load cell gets only vertical force transmitted from the thin washer as it is perpendicular to the load cell. So that is why I have the washer set to 1.002" from the shaft center. If the arm was not parallel to the load cell then the diagonal distance like you show has to be used to know the effective arm length.

As Don mentioned, the screw is parallel to the load cell, so I believe the setup he's using is correct. The load is being applied perpendicular to the load cell at 1" from the center of the shaft. If I were to use the dimension in the drawing, that would imply that the load is being applied at the angle you have drawn. I don't believe this is correct.

Also, thanks for doing this write up Don. I put quite a few hours into the 3D work and wasted a lot of plastic on prototypes, so in the end I was too lazy to do a write up like this with photos.

I like the use of Rulon J as it's supposed to be good for eliminating stick-slip. I made a new version that completely eliminates the bearings, but I might try revising it to incorporate the Rulon J before releasing it. It also has a sliding top so the batteries are easy access when they need to be changed.

Eventually I'm planning to make a handful of these torque meters to sell as a fundraiser for the junior team.

Hopefully this thread will help people out. I am glad you did the 3D stuff. I am not strong in 3D CAD as we design in 2D 99.9% of the time due to the nature of what we make. I have no issue visualizing, its knowing the ACAD command to do it :-)

I like the sliding top cover idea. I think the only thing I would want different based on my build would be for the c-bore that the forward sleeve bearing goes into to be about 1mm shallower so I would not have needed to add that additional spacer between the stop collar and the Rulon J sleeve to make sure the washer cleared the front of the case.

Thanks! Yeah, a picture is worth of a thousand words, indeed. I was thinking about the math in the way to wrote it down, but did not have the time to do it yet. Now I do not need to.

Yes, I stand corrected, I think your math is correct. And luckily this also explains why my meter readings are so low, I have always wondered how people han wind their motors to such huge torque readings, while my motors always break at much lower TQ. My meter is adjusted the wrong way, and as I am using gram*centimeter, my arm is much shorter, and hence the ratio L2/L1 is much larger, giving a considerably higher error.

This is a really nice device. Well done on the design and development.The only issue I can see is that the particular electronic scale chosen doesn't seem to be available in Europe...I'll keep looking for a source.

I'm not sure how bad shipping would be, but I could send you one. They cost me $9 delivered. I would only ask for that plus the cost of shipping. Or I could sell you a complete torque meter with profit from the sale being donated to the US junior F1D team.

The only issue I can see is that the particular electronic scale chosen doesn't seem to be available in Europe...I'll keep looking for a source.

Skymon, the Amazon link Don posted above claims that the scale will be shipped to UK or any European country as well. I'm sure you can also find the same scales in any Chinese web store like Aliexpress.com

Thank you Don for an informative build description. Good to have an answer for the torque arm measurement issue that I have discussed with Tapio a few times:). My meter is a Bill Gowen-style with a plywood frame. It is a good alternative to 3d printing if you don't have a 3D printer (and skills to use it) easily available.

BTW, can you change the batteries in Jake's design without removing the axle?

Thanks Don,an excellent tutorial, almost as good as your description a few months ago of how to make a No-Cal fly for minutes on end! There was one little point, brought to mind by your discussion withTapio about the precise length of the load arm. I have a feeling that on these cheap scales they give slightly different values depending on where the load is applied to the load cell and that the precise centre is not necessarily,consistently the best. I am not trying to make an issue out of this, I don't think that checking torque on a rubber motor warrents thousandths of an inch but I just wondered if you had come across this aspect.John

Generally these "binocular" load cells (binocular refers to the dog bone shape in the middle) are single point load cells. What this means is that the measured force is insensitive (to an extent of course) to the position of the applied load making them ideal for weighing scales with platforms. So as long as the torque arm length from the shaft centre is accurate (as has already been discussed) the actual point of contact on the Load cell need not be accurate and you will still get good readings.

BTW, can you change the batteries in Jake's design without removing the axle?

Simo

It's a bit tight, but you can change the batteries without removing the axle. The updated design I'll be publishing soon is also tight, but it too allows for changing the batteries without disassembling anything.